Nice on checking to make sure there were no more fish and if you did not see anything else dead, it could be a bucnh of gunk and the tank is now playing catch up.I checked every day after another fish died to make sure nothing was under the rocks. Then once all the fish died I rearranged and checked each rock to make sure nothing was stuck or wedged in between. As far as Brittle Stars or Bristle worms I Can't check now because I am not home but all the time moving the rocks I didn't see anything fall out outside of the sand.
As for the test I tested my tap water to make sure it was accurate and it read my tap water as 0 ppm ammonia so I don't think the test is wrong.
The only thing I could think of that could cause this issue was the sump had a lot of detritus in it and the protein skimmer needed to be cleaned. So I spent all day Saturday thoroughly cleaning all that stuff out .
If there is a petco near you, pick up a seachem alert badge on your way home as @MnFish1 stated. It will take it a bit to register once in the tank, but can be used as a secondary test to make sure the tank is safe. They are only a few bucks and handy to have.
I would still do a swish test, to make sure a bunch of crud is not in the rocks. If it is, you can swish all the rocks out of the tank using old tank water to do it. This can help speed things along, as if too much stuff sits there it can prolong the process or become an issue if something knocks a bunch out all at once. Just a rock on top will suffice and will not disturb the tank too much.
Lots of variables here, but a few simple things can be done to check to see if it is an issue or not an issue.