What is a silent heat ?

My dog got out of the house yesterday with my other one, One od them returned and the other one did not, When we did find her after looking for a day and posting /handing out flyers we found her, When she came here her private area looked pretty red , It was not blood (I bathed her and there was nothing) I am wondering if a dog goes into heat and was always blood before can they suddenly go into a silent heat ? As well if she is not in heat there is no way for her to be pregnant right ? I have never had a dog get out of my house before and this is really new to me. Please give any advice even signs that she is pregnant. I really am not ready for it. I did not plan on it and I forsure not going to breed her. I had just got her from a lady who had her to breed her and did not lok after her at all. Please help me out with advice.