I'd be really freaked, too OP.
I believe you have looked and it is not there.
Please answer a few questions--
Please - really answer these:
It seems you have a private key code -- this is a code you set yourself, and have told no one, nor have it written down anywhere, correct?
You have had no maintenance or tradesmen in your home during this time, correct?
Grocery deliveries?
Have you assertained that there has been no maintenance (emergency or otherwise) anywhere in the building (not just your own flat), since you last used the ironing board?
They should have informed you of any building maintenance, but do inquire, to be sure.
These next two Qs are actually important:
Did you put the board away in the cupboard after you last used it, before you left for the day?
Or, was it still standing, when you went out the door?
Even if it was standing out when you left, but then not there when you returned home, you may not have realised it went missing from that first day, since it is not usually there, anyway. (Or even for a few subsequent days, for that matter.) You'd've assumed it was in the cupboard.
Any creepy men (or women) that you've noticed lately? Anyone who gives you even the slightest twinge of yuck factor?
The concierge staff concerns be a bit.
And change the keycode! (If you haven't already.)
Everyone seems to think this is funny, and it would be, if it were simply misplaced; but am a bit creeped out for you, OP.