I used to find all sorts when I was a gardener for other people.
Secateurs were regular, but also the odd phone and penknives
Folk chuck them in the bucket whilst weeding and forget about them
Those little windows from official envelopes. It doesn't matter how zealously I tear them out, there are always some left. They make a horrid crackly noise whenever you unearth one.
I found what i thought was a snake when out checking for slugs around 11pm...Think it was a slow worm....S Myself lol ..didnt stop me videoing it.
Secateurs. Brand new Felcos when lost by daughter. Now they need a new blade.
Trowel. Left by OH after cleaning grass from under the Flymo.
Field mice nesting.
I discovered a wasps nest under construction in one of mine several years ago so luckily I was able to disrupt their nest building before it got out of hand and became a nuisance. Just the other day I lifted the lid on one of my compost bins on the allotment and disturbed a mouse which ran around and around before escaping down a hole in the half composted material, usually I give short shrift to mice but as it was on the allotment and I already knew that mice regularly live in the compost bins I let it get on with life but I do wish that they would stop eating the worms as the compost would decompose much better if it was full of worms.
I didn't find what my compost bin had hidden in it for months after I mulched the borders ......thousands and thousands of stachys seedlings. Won't put them in again.
Rats! Sorted the problem, but have found slow worms nesting (the eggs are fascinating), voles, ants.
Also sympathise re the seedlings: years and years ago a "friend" gave me some seeds: Nicandra. At the end of the season, I composted the plants with what I thought were unripe seedheads. I am still finding the plants popping up, over a decade later!