So we do sometimes expire old tab data. If a device hasn't had any tab data changes in the last 14 days, we start to consider the device inactive, perhaps you bought a new phone or it broke or something, and we remove all of the tabs from that device. However, we never do this piecemeal to the tabs of a device. If three tabs are recent and the other 14 are a year old, we should show all of them. So this alone doesn't explain what you're seeing.

I'm curious if there's potentially something odd happening on your phone, where some of these old tabs, while they show up in the tab browsing UI, maybe they're only partially loaded and Sync can't access them? I'm not sure. What type of phone do you have and what version of Chrome was it running?

Phone is a Nexus 5X running MHC19Q. Chrome is 50.0.2661.89.

The phone and chrome are in daily use.

Just now I opened history on the desktop and there were no devices listed.
I opened chrome on the phone an suddenly I have 3 tab again listed on the
desktop (they are from today).

Desktop chrome is 50.0.2661.86 (64-bit) but it wants me to update. I will
and I'll see if that helps.

Just to make sure, these tabs don't have urls like chrome://<stuff> or file:///<stuff> right? Because we also exclude those kinds of tabs from tab sync.

To make sure I understand the behavior you're seeing right now, after you saw 0 devices on your desktop, you then opened chrome on the phone, but then checked again on your desktop's history page. And that's when/where you saw 3 tabs for your phone, which should still have 17 tabs?

Yeah, these are regular tabs.

Here's a screenshot from desktop and mobile.

https://goo.gl/photos/dGiZLPETgjB9EVcT7

BTW, the 1 tab that's showing on desktop now is an old one that wasn't
there earlier however I noticed that when closed some other tabs earlier,
it became the top-most and chrome seemed to re-render it (in the tab
switcher view). That seems to have caused it to appear in the history. So
it really does appear to be a case of tabs timing out individually.

This sounds like  bug 565481 , which I still haven't been able to track down the root of, although I haven't had much time to investigate recently. It appears to be specific to Android though, and doesn't happen on IOS or desktop.

Will try to pick that bug back up (or hand off to someone with more cycles)