Possibly not what you want to hear, but specifically now you've said you're a contract cleaner, I would say this stinks and smacks of the supermarket wanting to either renegotiate the contract or go elsewhere and they are attempting to either gain leverage for renegotiation and/or don't want to TUPE employees like yourself who may quite easily be on much better terms than they desire in 2019.
Speak with your legal adviser about the information being disseminated. Unfortunately some of that could be that they've gone on a fact finding mission (rightly or wrongly) about your conduct. It could also simply be that someone has added two and two together based on what they've seen and started the gossip chain or simply told you in order to wind you up.
Please be very careful how you handle this. Be professional and be very clear that you were doing your job, you have no recollection of the event and anything of intrinsic value would be passed to Customer Services/whatever the company directs. It might be helpful to compile (for your future use) a list of times you have handed in things with an obvious value - wallets, phones, cash, jewellery.
For what it's worth, if the packet wasn't sealed, it would not occur to me to do anything other than bin it in your position. In fact I would probably have binned it as a supermarket customer unless it was obviously new. If I had found it on the street whether sealed or not, I would have binned it regardless.
Thinking on, do you have any contact with colleagues in other stores assuming they work for the same contractor as you? I just wonder if there's others that this might apply to, particularly if they've been long-standing members of staff. It may not as it may simply be that individual store is embarking on an attempted cost cutting exercise, it depends on the supermarket chain how localised they are about things like contract cleaners/security.