There are a number of really useful online resources, both Free and Paid For, where you can research an individual or even a company, to see if an email address, mobile number, a username etc, have been linked to any hacked details. These aren't usually individual accounts being hacked, but more often relate to large scale hacking events or cyber-attacks, where a company or online site has been hacked, and millions of account holders details illegally obtained and then sold on or leaked online.
So, you could search via a name, address, email address, mobile number, or even search via a password, to locate linked account details, which may open up your investigation or research. So you may search via someone's known email address and as a result locate a linked mobile number or address.
As well as linked email addresses or mobiles, the results may provide other useful leads and additional information into a subject, as well indications of which online sites they were linked to when hacked, and so where they may still have an online account etc.
Hacked Email & Online Accounts Checking Sites
Customer Service Details (if your online account has been taken over)