Weird question maybe??

This little man is nesrly 15, a little senile, but still very very active. I like to have him outside with me and my other dog when I'm working (I have a croft), but it's really hard to keep track of where he is. The veg gardens he's in are pretty enclosed, but its still about an acre of trees, bushes and veg beds, and his colouring make him Invisible! He's also very deaf and pretty blind, so I can't just call him over when I lose sight of him. Honestly, I wish I could tie a helium balloon to his harness so I could see it bobbing about 😆 I'll probably try to find him a fluorescent coat, if I can get one that's lightweight enough for summer, but is there something out there anyone's made specifically for this problem? Like a lightweight flag or something? He's only 6lb so I don't want to make him wear anything cumbersome. Any ideas?

(Just to add, we're in the UK, nothing is going to eat him when I can't see him. I mean, the chickens might have a go, but yeah, no coyotes or anything :) )