I had searches I didn't do show up on work devices, and ended up losing my job over this, for a first infraction. I know 100% I didn't search those things. I had no screen locks or passwords on any of mine or work devices. So it's possible anyone could have got on them, without my permission.
I never even knew I was required to have screen locks, not did I care if people used my devices. I never let anyone use my work devices. They said illicit or inappropriate use.

But I know 100% I didn't do it. How can I prove if it synced vs actually searched from that device? There has to be a way. Is there anythign I can do?

I know I signed into google maps with my personal gmail, but I can't remember if ever signed into Google or Chrome on them. But I know I was logged into other work devices too. I cannot remember. I'm Google dumb. About all the settings. Have anxiety and never read the pop ups. Just clicked ok. Assume something synced from another device.

The only real advice anyone can give you is to contact a lawyer and see what they say.

You do have a basic responsibility to keep work devices and data secure, so not password protecting them is fairly irresponsible of you.

In future, keep your work and home devices separate. Don't sign into your personal stuff on work devices and vice versa. Always lock your devices when you step away from them to prevent others from using them without your permission. And don't ever search for porn, drugs or anything else which could be possibly frowned upon on a work device or on the work network.

Someone else was probably on your machine

In a lot of employment contracts, you are considered to be responsible for anything done under your corporate account/devices even if you yourself weren't at the keyboard.

It is unlikely that this was caused by syncing your Google Account with Chrome. Chrome has a tendency to identify what devices conducted what action. Plus the ability to synchronise histories would prove that the History data can be altered, and thus would typically not be trusted for the firing of an employee. Regardless, this was still a bad move.

What probably happened is that actual attempts to conduct those searches by your account and/or devices was picked up by network security devices as it was being performed. You'll notice I said 'by your account and/or devices' and not 'by you'.

In a lot of places, you'll see a culture for pranking people who leave devices unlocked. Most of the time, this is just a harmless 'beers are on me email' but more juvenile/malicious people may do things that stir up a bit more trouble, unaware (or possibly fully aware) of what their actions might bring.

I had no screen locks or passwords on any of mine or work devices. So it's possible anyone could have got on them, without my permission. I never even knew I was required to have screen locks, not did I care if people used my devices.

Locking your devices and accounts should not be something that needs to be spelt out. It should be, and needs to be from this point on, common sense. Your devices likely hold a lot of company-sensitive information. What if someone got onto your machine and accessed something they shouldn't have access to? What if someone trespassed into the building and got access to your computer? You could have been potentially looking at a lot worse than just getting fired for inappropriate searches.