PA31's have a real issue with their gear setup.
If there's a bit of stretch in the cable between the gear selector and the hydraulic control valve, you CAN get a situation where a normal selection results in the cycling of the gear doors, but no gear transition.
At the conclusion of all this, the gear handle returns to the normal "gear-down completed" position.
The only give away that this has happened is the lack of the clonks that usually accompany the PA31 gear transition, but if you miss them and there's no mirrors...it's only the gear lights left.
What makes it worse, is that the gear warning horn is usually set to about 12 ins MP which is usually only realised during the last stage of the flare.
If you're one of the people who routinely return the flap selector switch to the neutral position after selecting full flap (as I do to mitigate the risk of a flap motor over-run in the event of a flap-limit switch failure) then you have just inhibited the gear warning horn altogether.
So it's not too surprising that there are a number of PA31 gear-ups.
Not all are necessarily "forgot to lower the landing gear".