Unfortunately, in most places the cops won't care, so unless you want to go vigilante (unwise) your best bet is insurance. Organized thieves will be on to it anyway. The real solution is probably to get manufacturers to use serial numbers. There's a reason bike theft is like 10 times more prevalent than motorbike theft, and it's not the weight...

I use an Apple Airtag mounted in a bicycle bell housing. You can buy those on Amazon, they’re quite cheap. The Airtag bell looks just like my regular bell. So, I just put it on the opposite side of my handlebars. Means I’ve got two bells, but that’s not uncommon on mountainbikes anyway.

You can track the bike using your iPhone and the Find My… network.

There’s some caveats to it, obviously. You need to have an iPhone for one, and the tracking works best if you’re in an urban area with lots of other iPhone users nearby. It’s not an actual GPS tracker, but rather gets pinged by bluetooth from nearby phones which report the location. Another caveat: they start to chime after x amount of hours away from your phone, as an anti-stalking measure. But you can just disable the speaker on them if you want to.

The overall benefit is that they’re quite cheap and require no additional data plans or sim cards.

As others said, Tile or AirTags would probably be your best choice. AirTags have a better chance of being located (due to the large amount of iPhone users) but you'll need an iPhone yourself. Tile should work too if you are in a more densely populated area. I have an AirTag on mine, but it hasn't been stolen so I don't know how useful they really are. But there are tons of ways to hide them on your bike. Just look for "AirTag bike mount hidden". Under your bell, under the saddle, behind a reflector or even in the fork. If you want to go maximum security, disable the speakers (you'll need to cut the AirTag open for this) and use 2 AirTags in different locations. The thieve might eventually be notified that an AirTag is moving with them, but you can hope that you'll find your bike before the thieve found both of your trackers. Good luck with securing your bike!

You're basically stuck with either Tile or Airtags. Most likely will see more success with Airtags, but if you're in a dense area, Tiles would probably ping as well.

Edit: Apparently Amazon is partners with Tile. Sooo pick your poison. Let me know what you pick, it could be useful for me as well. I have an old iphone 7 that could be used to set one up and my old apple account still works. So I might be able to set up the airtag thing in the future if need be.

I've heard of people doing this with Airtags and recuperating lost bikes successfully.

Honestly, I don't want to track my bike down. A thief who is sophisticated enough to get past my lock is probably sophisticated enough to look for a tracking device, so if I find it, it's probably already parted out or tossed in a ditch somewhere. I'm not going to personally track my bike down (seems dangerous), and the police are unlikely to make it a priority. And if I felt the tracking device would help, I'd probably let my guard down because I have a backup plan.

Instead, I make my bike unappealing to steal. I use a high quality lock, avoid parking where thieves are likely looking, and cover up anything that could indicate any sort of value.

Register your bike, with the police and a registry like 529 Garage.

The fact is, police recover hundreds of thousands of stolen bikes, but can't return them to the rightful owner because it hasn't been registered.

FWIW, I use two locks, and a hidden motion alarm when I park my bike. I only have a tracker on the keys for the locks/alarm, but will likely put a Tile somewhere on the bike this year.

Also, keep in mind that thieves are scumbags and will steal anything off your bike: lights, saddle, pedals, accessory mounts, etc.

Do your best. Unfortunately, a motivated thieving scumbag will still try, and if they can't steal your bike, they'll try to damage it.