you'll see debris where the ship exploded. mats will float in the vicinity, possibly drift a bit.
in any case they appear on your contacts panel (to your left) if in sensor range, you can target them from there to locate them, but:
if you fire a limpet with something targeted, it will scoop that up and self destruct afterwards
if you fire a limpet with nothing targeted, it will keep scooping up anything in range until it 'expires'
also, open your cargo scoop, and make sure there are no obstacles right below your ship, limpets are dumb and will just smash into them.
in any case they appear on your contacts panel (to your left) if in sensor range, you can target them from there to locate them, but:
if you fire a limpet with something targeted, it will scoop that up and self destruct afterwards
if you fire a limpet with nothing targeted, it will keep scooping up anything in range until it 'expires'
also, open your cargo scoop, and make sure there are no obstacles right below your ship, limpets are dumb and will just smash into them.