I have been called on several 'towel ingestion' incidents over the years. A few 'panties ingested' ones as well, but the towels are usually the tough ones to get out.
I have had great luck with sucking them back out the way they went in. You have yours out already, but for others who find themselves in this predicament, here are some tips.
First: fill the tub all the way to the top. You are going to do things that will work better with more water in place.
Buy, borrow or steal a pool vac hose. If you can’t get your hands on one of those, try a shop vac hose.
Get a siphon going in the above-mentioned hose, and stick the end over the hole down which the towel disappeared. If it only flows for a short time and then stops, lift the end of the hose off the fitting and establish water flow again. Then put the hose down on the fitting suddenly. You may have to ‘jar’ it loose.
Don’t be surprised if you have to try several times, even to the point of having to add more water to the spa. The towel may just come out to the point of being visible but not all the way. In that case grab it with pliers and gently WORK it out. If you simply pull you may rip off the one little part and not be able to get the rest out.
I hope nobody ever needs these tips !!