The main issue here is not about the cost of the plug BUT stealing. They are theft.
Why would you assume that it was "stolen" rather than just accidentally packed? A basket waste is highly unlikely to fit the sink in their next place, so it seems an unlikely item to want to "steal".
I've used many removalists over the years, and on arrival at my new home, have often discovered to my annoyance that the removalists have packed plugs, door bells (which they had to remove from the wall), and all sorts of things which I had assumed it was obvious were supposed to stay with the previous property.
Yes, the departing tenant should still return those items, to be cooperative, but given that the basket waste wouldn't usually be specifically listed on the condition report, the tenant could easily say "it wasn't there when we moved in", and you'd have Buckley's of winning a claim for the cost.
Given your position, I'd be asking the tenant nicely if they accidentally took it with them and would remind returning it, please. When I was a tenant, I know if a former landlord had remotely implied that I "stole" those items, my desire to cooperate would evaporate, and the chances of the item being returned would drop dramatically.