Improper Investigation from opposing attorney

What is the name of your state (only U.S. law)? Maine

Can an attorney, who is doing his own investigation for his client, call people and inquire on the person whom his client is suing, without ever identifying themselves as the opposing attorney or who they're representing?

Basically, can an attorney act as an anonymous private investigator when speaking to related parties in gathering evidence against the party in which he is the opposing counsel in a legal battle?

Yes....but. There are certainly some people he can't contact (like a represented party) and he cannot misrepresent his position/identity. Most would not do so to gather evidence as there would be a risk being called as a witness themselves.