It depends on state law, but the unauthorized access of someone else's account can be a crime. Having a password does not give one the right to come and go as they please. If I give my neighbor a key to use for emergences, that does not give them the right to come and go when they want to. The same applies here.
I have not researched the issue with regards to IL state law, but it is technically illegal in most states I am aware of, though it is rarely if ever pursued simply because it is hardly worth the effort.
And without proof, there is really nothing the police are going to do about it. They are NOT going to seek subpoenas and/or search warrants to try and track this down. They just won't.
And there is this:
(720 ILCS 5/16D‑3) (from Ch. 38, par. 16D‑3)
Sec. 16D‑3. Computer Tampering.
(a) A person commits the offense of computer tampering when he knowingly and without the authorization of a computer's owner, as defined in Section 15‑2 of this Code, or in excess of the authority granted to him:
(1) Accesses or causes to be accessed a computer or
any part thereof, a computer network, or a program or data;
Seems to be a crime.