Been there.. done that also! Next...?
I'd first review " the" purse".
Check for a separated lining perhaps mistaken for a pocket ,or envelops rolled under it.
Not knowing her bill paying method ,it would still be next.
I write on the bill the date of payment,amount ,and method. Then the bill ( minus coupon that gets mailed) gets put back in envelope received in then put on a pile that gets stored once a year for records. That pile would be where stray ,or misguided envelopes would end up.
If for some reason the closet purse location was not desired to be reached that night, from where she was counting and doing other things ....where would a temporary stash be made? Under the mattress ? In a drawer? An overfilled drawer can have items dragged off the top and they end up behind or along side, or below the drawer.
Between mattress and head board is just one pinch point that can grasp envelopes if they were left on the bed. Perhaps to eventually get knocked on the floor under when making the bed, I have no idea whatsoever how a paycheck of mine ended up under the bed.
I had reasoned( justified?) how the missing funds had been spent knowing I was short , so finding it was interesting.......
Lastly ,without knowing if depositing it was on her mind ,could it have been placed for either of you to take it to the bank the next day ,or soon? Where would she put it then? Your coat pocket ,on top of the fridge , some other relay spot where things go when leaving is planned?