How does plant scale infestation get to inside plants.?

I had some indoor plants that I had gotten from a nursery that developed scale and spider mites that I ended up pitching. I have 3 small lemon trees that I started from seed. They have always been inside, were potted in new pots with commercial potting soil and as I was caring for them today, what did I see - scales. They are in the same area as the old plants were, but I washed and bleached the area and several weeks passed before I potted the new plants. How do I get rid of them and keep them gone.