Interestingly enough, I did some math here:
Take daymar, it has ingame radius of 295km, which conveniently gives us 1.09mil sqkm surface area.
If you are landed on the planet mining with 10k EM rating and some guy with a ship with s3 radars flies to the planet and stops at 3km height, (this means they'll spot you if you're 9.5 km away along the surface), they'll have 0.026% chance of happening by where you are.
Meanwhile if you had 30k EM rating, spot is 29.85km and it'd be 0.26% chance, yes, 10 times higher for 3x EM. Still abysmally small though, but maybe if they flew around the planet.
The circumference is 1853 km, so if they did a full swirl around the planet, they'd have 1.03% chance of finding a ship with 10k EM and 3.22% chance of finding a ship with 30k EM (interestingly, the more tenacious your opponent is in circumventing the planet or moon, the less benefit you get from having lower EM rating, though even the most tenacious opponent produces a linear chart here.)
In either case, the odds of being found on the surface of the planet are pretty slim. Unless literally the whole server came to search you and paced themselves out evenly, then circumvented the planet you are on.