I thought that but as far as I know Hercule Poirot doesn't - he smokes small cigar things (cigarellos?)!
I happen to work with two avid Hercule Poirot and Sherlock Holmes fans but neither of them - or me - know much about Maigret. So far we have discounted
Pipe smoking and un-married (Maigret is married, the other two aren't.
Two other possibilities we have come us with are
Brothers - i.e. Mycroft Holmes and Achilles Poirot (thought not sure if it is Hercule's brother or Hercule himself in 'disguise') but can;t find any reference to Maigret having a brother. Likewise a 'helper' who has connections to the military (i.e. Dr. Watson and Col. Hastings) but, again, can't find similar with Maigret
Hope someone comes up with an answer soon as it is driving the three of us mad!