I figured it was time to do some research, so I travelled up Route 495 to the Seabrook, NH visitors' center, and indeed WMYF was in fact on the air, but with a weaker signal than I recall at that spot. Previously it boomed right down the coast and overwhelmed WFEA-AM 1370. Today, I received fair-to-middling signals from both WFEA and WMYF at that location. Travelling back south on Route 495, WMYF disappeared totally near the Merrimac (that's the way the Town spells it)/Haverhill line. So they're either operating at reduced power for some reason or running a directional antenna even during the day.