OK... I did some more testing and figured I'd report back to make an ending to this post.
I'm pretty sure it was the shrimp that spiked my phosphates. As a test, I took a half of one of my left over shrimp from the bunch I'd bought that I'd stuck in my freezer. I thawed it, and plunked it into a glass of fresh water. I'd say I had 1/2 shrimp in about 12 ounces of water. The water tested 0 to just a trace of phosphates (just a mere hint of blue with the Salifert test) before the shrimp was added.
After just 24 hours of the shrimp sitting in the 12 ounces of water, I tested again. Wow. I didn't know that Salifert test could turn that blue! Off the chart. I used a Tetrafin phosphate test that I had (that I don't like because it's just not very sensitive at lower ppm
levels like the Salifert is) that measures up to 5ppm. Off that chart too!
Now granted... I've got a half a shrimp in 12 ounces of water so my concentration of phosphates in the water is no where near what it would be in even a 10 gallon tank. I'm waaaaaayyyyyy more concentrated with my test method. However, there's no arguing that the phosphates are coming from the shrimp, and at a pretty good level only after 24 hours.
Next time I find real fresh shrimp - like directly from Bubba Gump's boat on the dock - I think I'll repeat the test. I'm guessing that there's probably phospates in whatever they spray/coat/dip the shrimp in to make it stay fresher longer.
Folks might want to consider this when they get their shrimp for their cycle - even if the shrimp looks fresh, it probably has been previously frozen somewhere along the route. And if it has, it's probably been given a spritz of something to help it out. So if you find phospahtes somewhere after your cycle, and your water tests fine, don't pull your hair out trying to find the source. It's probably gone already!