Found one cockroach nymph every 3 months, how can I find the source and prevent infestation?

Early October last year, I found a cockroach nymph on the wall and I got rid of it. I was worried about infestation, but luckily I never found anything after that until today. I saw another one on the wall, near the spot I found the last roach. There is a big window nearby but I haven't opened it for a long time. I put a black cap near the spot I found it. Is there anything else I can do? Has anyone experienced similar thing where you only found one nymph?

Edit:
It seems that the type of roach is smokybrown. They are primarily outdoor insects.