All the bits I've made, such as bumper and other parts have my initials and/or postcode welded into them .. not always where they'll be seen ..
When it's parked it's usually at home all week or month, depending how good the last trip out was ..
and I then leave the bonnet up with bits off it, or on ramps, or a wheel off and locked away. It's also behind two locked gates and another parked car etc, not that they'd be much good for determined thieves, and I also have other passive locking methods employed too ..
It's also usually covered in mud, looks a right state with foliage hanging from the axles, and looks like parts wouldn't be worth selling from it.
I know a lot of the local scrotes too (School caretakers get to hear all the stories) and make a point of stopping, kicking a wheel or something, then finding it hard to start ... or when parked at the shops, get it to develop bad starting and a cloud of smoke on start-up.
Detering them seems a reasonable idea to me before they even think about trying to unlock the doors ... They don't want to clean them, they want to strip and part them out with little effort .. make that effort too much and they might move on. Silly things like odd stickers. I hate stickers, but I've started putting odd ones on, they're unique, I KNOW WHERE THEY ARE with photographic evidence of them all, so any parts might well be recoverable. Even if time is taken to tear them off the adhesive can leave imprints and marks!
OK, some poor sod down the road with a shinier, cleaner, landy might get theirs nicked instead, but he could do similar ..