@Squid - good catch! And I hope your kitty is OK now. (((Hugs, hugs, hugs)))
Can you just clarify which meter you're using? (Sorry if I missed that).
If you are using a pet meter, such as Alphatrak, numbers below
65 mg/dL (3.6 mmol/L) can be hypoglycemic.
(FYI, cats hanging out in closets can actually be a symptom of hypo, especially if that's not normal behaviour for them...)
And yes, it is perfectly possible for a hypo to have NO symptoms until the situation is severe. Some cats show symptoms early on, some when a hypo is moderately advanced, and some do not show any symptoms until the hypo is severe. 'Absence of symptoms does not mean absence of hypo'.
Be aware that if your kitty has had a hypo he may subsequently be more sensitive to insulin, so do watch his BG carefully over the next few days. (Although chances are that he will initially 'bounce' up from this very low number).
For those new to using Vetsulin the general advice is to try to not let the BG drop below 90 - 100 mg/dL (5 - 5.5 mmol/L) in order to give some buffer of safety. That's just until the caregiver has got enough data to get some understanding of how the insulin is working in their kitty.
There is some information about using Vetsulin on the following FDMB link.
Beginner's Guide to Caninsulin (Vetsulin)
Eliz