Warum muß man etwas feststellen? Man stellt einfach fest.

Werner, das hat etwas mit Pragmatik zu tun. Im Satz mit 'mußten' schwingt mit, daß der Sprecher recherchiert hat _und_ dabei auf eine ihm unliebsame Tatsache gestoßen ist. Das bringt das einfache 'feststellen' nicht zum Ausdruck. Insofern bin ich mir auch nicht ganz sicher, ob Dein Übersetzungsvorschlag diese Konnotation besitzt.

Oder: "It has come to our notice that..."

we have, (unfortunately,) determined that this account has been closed in the meantime

Dann müßte dort etwas völlig anderes hin wie "we regret having to inform you that our investigation ..." oder so etwas.
Ich schlage vor, daß uns js1977 jetzt den Kontext liefert, der zu einer endgültigen Übersetzung taugt, denn ich vermute immer noch, und dies bis zum Beweis des Gegenteils, daß das "mußten" einfach nur antiquiertem Beamtendeutsch zuzuschreiben ist.

Kleine Ergänzung: Schon die Tatsache, daß die Feststellung der Kontenlöschung z. B. auf einer (Kosten verursachenden) Rückbuchung beruht, kann in meinen Augen den Gebrauch von 'mußten' rechtfertigen.
CM2DDs Vorschlag drückt das IMHO sehr treffend aus.

Based on Peter's explanation, I would say:
"We were forced to conclude that the account was closed in the meantime."

Abra, I'm sorry to say that you seem to have misunderstood my explanation at some point. It's not about concluding, but about "establishing a fact which then is stated."

Ich möchte da kein "unfortunately" hineinbringen, denn das ist zu wertend und erscheint so nicht im Ursprungstext.

Werner, wie bereits gesagt, besteht unser Chef darauf, dass da nicht steht: "wie wir herausfinden mussten", sondern "wie wir feststellen mussten".

I've often seen "feststellen müssen" used to express regret or irony; if that is present in the text, then using a word like "unfortunately" expresses that, it doesn't add anything.

"As we had to find out" doesn't work, as if we used it, it would mean "because we had to find out" or "since we (still) needed to find out". If "unfortunately" is no good, then just "as we found out" would be the next best thing.

"Find out" is a synonym for "discover" and is one translation of "feststellen".

Ich weiß, Chef ist Chef, aber vielleicht kannst Du ihm sagen, daß Ihr überhaupt nichts herausgefunden habt; das war nämlich die Bank, die Euch dann informiert habt, und Ihr könnt das bestenfalls erfahren (Alternative: as we had to learn).

PLEASE don't use "had to". it's utterly wrong.
But the right phrase might occur to me in a minute

@Peter: I guess my brain is just in the pre-lunch lag, but I don't really see the difference. to conclude = "to determine (by reasoning)" (def'ns 4 and 5 in the first link).

The phrase "to be forced to conclude" is relatively common in "Beamtenenglisch" and means that based on research the person/body has no choice but to determine that XXX (an outcome which they may not have wanted), which is basically what I understood you to say. The second link has an example of its usage.

js1977, könnte Dein Chef per LEO-Eintrag überzeugt werden, daß es ein Wortpaar 'feststellen - to find out' gibt? Und vielleicht überzeugen ihn die beiden muttersprachlichen Vorschläge ja davon, daß das eben nicht nur 'herausfinden' bedeuten muß.

@Werner: Genauso eine Wertung steckt im 'mußten' - neutral wäre die Formulierung 'wir stellten fest'.

I would say "find" -- it is not the same as "find out"

Unfortunately we found that the account had been closed in the meantime.

...had since been closed.

We regret to say that we were unable to transfer the sum in question because the account has (since) been closed.

We were informed by the bank that this account has been closed...

I assume this meaning of "Feststellen" is the one your boss means?

festellen "1 b) bemerken, erkennen, wahrnehmen: eine Veränderung f.; man stellte einen Einschuss am linken Oberarm fest;" (Duden)

The Leo entries which fit this meaning of "festellen" are:

find (out) - you're right, Ghol, "find" alone sounds nicer.
establish
ascertain
determine

"Detect" wouldn't fit here, as it means it was very hard to discover the information you found:

de•tect /dtekt/ verb [vn] to discover or notice sth, especially sth that is not easy to see, hear, etc.: The tests are designed to detect the disease early. an instrument that can detect small amounts of radiation Do I detect a note of criticism? http://www.oup.com/oald-bin/web_getald7index1a.pl

Abra, thank you for your comments and the explanations/definitions given. Maybe I haven't made myself clear enough, sorry. I see your point, but still am not totally convinced that - left aside the 'Beamtenenglisch' - to conclude is the best choice here. The salient point of my hesitation is the element of reasoning prevalent in 'conclude', whereas the main direction of "feststellen" in js1977's example is stating an established fact. Only the 'müssen' adds this element of 'coming to know by certain circumstances', which to my eyes conveys a certain disapproval, clearly expressed by "unfortunately".

good gracious, LEO is soooo slow in accepting entries :-(

Abra, thank you for your comments and the explanations/definitions given. Maybe I haven't made myself clear enough, sorry. I see your point, but still am not totally convinced that - left aside the 'Beamtenenglisch' - to conclude is the best choice here. The salient point of my hesitation is the element of reasoning prevalent in 'conclude', whereas the main direction of "feststellen" in js1977's example is stating an established fact. Only the 'müssen' adds this element of 'coming to know by certain circumstances', which to my eyes conveys a certain disapproval, clearly expressed by "unfortunately".

Like it, @CM2DD

I will, exceptionally, pat myself lightly on the back and refer to posting #4 in this thread, in which I suggested 'determine'....