Long time lurker, first post! Here's a table I made full of weird omens and even maybe a plot hook for your game.
Roll a d20 to get random omens in your game:
1. A PC makes eye contact with a mysterious beggar, who gasps and runs away. If followed, they disappear.
2. A raven caws angrily at the party before flying away. More are seen periodically, perching and watching.
3. A woman in the street drops an urn, which shatters. There's a moment of silence in the street.
4. A baby won't stop crying in a nearby basket. If the players investigate, there is no child. The crying is gone.
5. The campfire goes out in the middle of the night. There is no wind. It is difficult to restart as darkness closes in.
6. The flames of nearby candles flicker out completely, but a moment later they return.
7. The moon is larger and brighter than usual. It seems to dominate the sky, and cast harsh, pale shadows.
8. A wooden road sign is snapped in half at a crossroads. It lies broken in the dirt.
9. The clouds are gray and full of rain. And yet it does not rain. Though thunder rumbles, not a drop lands.
10. A PC wakes up in the morning with a cut across their palm that wasn't there before.
11. Animals go wild. Horses buck and have wide, scared eyes. Dogs bark fiercely and pull at their leashes.
12. There's a large crack in a PC's drinking glass, and yet it doesn't break unless forced.
13. A bird flies into the wall of a building and drops dead outside the door.
14. A PC is contacted by the Sending spell, but they only hear heavy breathing and crying.
15. The players hear distant, sourceless harp music. When pointed out or mentioned, it stops.
16. An old man has a heart attack in the street. He stares directly at one of the PCs as he dies.
17. A passed out drunk wakes suddenly and screams, clawing at his eyes. He runs and disappears.
18. A strong wind that was blowing all day suddenly stops. The world is still for a few moments before life continues.
19. A child chases an errant ball, only to trip and twist their ankle. The ball is not found.
20. The temple doors are tossed open by a violent wind until a concerned acolyte rushes to close them.
I use these little descriptions to tip off players that something is generally wrong. Feel free to add your own favourite something's-not-right-here omens in the comments!