I asked my DH as a males perspective.
He says while he doesn't generally discuss wanking techniques with people, neither him, nor anyone he knows has used a condom for a wank. And that generally most men try and so everything they can to not wear a condom when having sex with a woman, that to him it would seem weird to wear one for a wank.
I also agree with PPs, do you use condoms together? It would be easy for someone to get bored and horny and take one out of the drawers and give it a go.
I find it very unlikely he would specifically go out and buy condoms to wank in to.