Silly Question About Appendix A

As I've said, I'm going to use DMG's Appendix A to build the one page dungeons. Still, I can't understand the bit about secret doors. In Table V. C it says:

*Check once per 10' for secret doors (see Table V. D, footnote).

And in Table V. D it says:

*If a passage or door is indicated in a wall, where the space immediately beyond the wall has already been mapped, then the exit is either a secret door (1-5), or a one-way door (6-10) or it is in the opposite direction (11-20).

So, question: what is the probability of having a secret door within those 10'? It says anywhere as far as I know. Come on old grognards, that's your chance to impress my young mind with your superior knowing of the old stuff!