All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
In principle, the transactions of people are valid and permissible unless there is evidence which proves otherwise.
Ma’lamat Al-Qawaa’id Al-Fiqhiyyah wal-Usooliyyah reads, “Rule: In principle, contracts are considered valid. Meaning of the rule: the principle in contracts that are held between people is that they are considered valid and correct as much as possible until proven otherwise, so if a contract is between validity and invalidity, and there is doubt about it, and there is no factor that makes one outweigh the other (i.e. the validity or invalidity), then it is ruled that the contract is valid until proven otherwise.” [Abridged]
Therefore, based on the principle, there is no harm on you to benefit from the electricity that is connected to that room, unless you believe or predominantly think that the electricity is stolen, then in that case, it is not permissible to use that electricity. But just doubting this is of no effect.
Finally, we warn you against Waswasah (obsessive doubts), and we advise you to repel and ignore them.
Allah knows best.