OK, not sure 100% if CCleaner is the culprit... But...
On several occasion, I found all my passwords in Google Smart Lock had disappeared. I traced it back to being the case each time I had used CCleaner before on one of my computers.
I double-checked on both computers I use CCleaner on, and the option to erase saved passwords in Chrome is not ticked.
Could this be a bug in CCleaner?
I luckily don't rely too much on Google Smart Lock (I use Roboform as password manager) but I'm not allowed to install Roboform on my work PC...