More neutral:
* Eavesdropping
* Catching
* Hearing
* Picking up
* Hearing by chance
* Unintentionally listening
More formal:
* Intercepting
* Acquiring
* Gleaning
More informal:
* Bugging
* Snooping (implies deliberate listening)
* Catching wind of
The best synonym will depend on the specific situation and the tone you want to convey.