I would bet the charger as well.
I just put several different m401s on charge, So Ill have a look as well. They all metered out between 2.8-3.04 before this charge, and had been sitting a bit.
Something interesting to note, The m401 batteries seem to be always conducting?

I get a .04v read even when the switch is not activated, on every m series I have. They are the only batteries Ive found that read when "off/standby"..
Ill let you know what they top out at, in a bit.
EDIT: Smoore claims they are 3.7 rated, which should top out at aprox 4.19? We'll see..