You’ve two scenarios here.
1) he’s guilty.
2) he’s innocent.
If 1) and you go down and call him on it, he’ll go “no I didn’t” and you’ll be no nearer.
If 2) and you go down and call him on it, he’ll go “no I didn’t” and you’ll be no nearer, and look like a dick.
There’s any number of explanations. Perhaps he saw you drop it and stuck it behind the counter, someone else later ‘borrowed’ it unbeknown to him and returned it when it didn’t work so that it wouldn’t be missed / cancelled. Perhaps he saw someone else lift it from you and then chuck it away but “doesn’t want to get involved.” Perhaps he’s a thief or chancer and just stupid. Hey, perhaps you locked it out and didn’t realise.
Whichever is true, you’ve absolutely no way of proving anything other than “I haven’t seen my card in a week and someone’s tampered with it.” You can’t then arbitrarily point the finger and go “it’s him, I’m sure of it, look, he even looks like a wrong ‘un!”, not even if you’re really angry. It could have been in a hundred hands since you saw it last.
Get it cancelled, keep a close eye on your account and move on.