Apple Watch unlocking by itself

I have an iPhone 7 Plus paired with my Apple Watch, both running the latest software. At first I thought I was losing my marbles as without entering my passcode the watch was unlocking. This happens after I have had the watch on charge and put it on but not unlocking it. Some time later when I check the watch its unlocked without any input.

I can only this the watch being in close proximity to my phone is the trigger and its now happened several times when I'm at work and my MacBook is at home.