Supposing the owner installed really invasive spying tools - the cameras and microphones, and used those to discover the tenant played the piano. Then, I think the owner would be guilty of whatever invasion of privacy law or tort is relevant, but the tenant would still be in breach of the contract. It would be right to punish the owner and tenant for their respective misdeeds, not say that because the owner did bad the bad action of the tenant should be forgiven or ignored.
> I think the law says otherwise? Evidence collected illegally is (usually?) not valid.
Evidence collected by the state in violation of Constitutional protections relating to search and seizure rights of the defendant is generally (with, however, ever-widening exceptions) inadmissible in criminal trials.
Otherwise, illegally-obtained may or may not be permitted; it quite often is, though presenting it opens one up to charges relating to the illegal act involved in gathering it.
It's not valid for the state to collect evidence illegally. If it was then it would make sense to have any laws constraining how the state could gather evidence.
In this case it's not the state that's spying, it's the landlord, and the landlord is constrained by the punishments for violating the law against spying.
A slightly different spin on it, if the landlord witnessed a murder while illegally spying on his piano should his recording or testimony be admissible in court? I would say yes though again the landlord should face whatever spying penalties apply.
What if the piano had a piece of tamper tape and the landlord saw it the next time he was doing work for the tenant. The students thought it was fishy, but there was no time line associated. (Or for that matter, the tenant may have gotten drunk and spouted off about it in the biergarten.)
