What happened to the spider that was left in the apartment in the building where Yasmin's family lives in? The Doctor told the spider to stay and then close the apartment door and that was the last we see of this situation.
As in almost all of the episodes in this season so far, the "enemy" turns out to be an innocent victim of the real enemy, mankind. In the end, the spiders were just brought "home" and trapped, and we saw that their exponential growth (which would continue through their lifespan) would eventually kill them anyway.
The spider that was left in the flat would therefore either:
- have returned home and been trapped when the mother called them; or
- have died due to its abnormal growth before it was able to.
At the middle of the episode, we can see that spiders where very numerous around the hotel area. There is also a dead spider in Graham's house which died of starvation.
We should assume that now that the threat is known (just Earth mutated spiders who escaped), the scientific lab will just have to search for the remaining spiders found on the map, even the one in the appartment, before they grow the size of a car...