When we think of pocket watches, we often think of old relics carried by old men. And we suppose that Time Lords are "old men" in the sense that they are 900+ years old. When a Time Lord wants to hide, all he need do is use the Chameleon Arch and store his memories and biology in a fob watch. When he's ready to recall his identity, it's just a matter of opening the watch, that is, if you can get through the perception filter.
Since you're all human and not Gallifreyan, this watch won't be much use to you for the whole memory/biology storage business. It's just a very spiffy recreation of the watch that The Master was stored in when he was living life as Professor Yana. Open it up and press the button for an out of this world nightlight feature. Just promise us you won't decimate the world's population... no matter how cool the word "decimate" is.