I don't really think a true false heat--when a cow is pregnant and will stand to accept a bull's service--is very common at all, but I have seen it happen...one time. That was a yearling heifer we AI bred then turned in a small lot by the barn with a herd bull for clean up. She did show heat, and he bred her, so we expected a calf from that natural service. She had twins 10 days early to the AI date...I'm not sure if it was a hormonal thing because of the twins or what. They were both heifers, but clearly not identical, for whatever that's worth.
I HAVE seen cows or heifers ride or try to mount other cows when pregnant when no standing heats occurred. I think that kind of activity IS pretty common, and while mounting other cows is a sign of heat, I don't consider that activity in and of itself, signs of a false heat. That's just normal activity. So if the cows weren't standing, then you may have a reason to be optimistic. If it was pretty apparent they either weren't standing YET or they may have been on their way out of standing heat, then I expect they were open for whatever reason.
I have a first-calf-heifer that was confirmed pregnant with a BIOPryn test back in June. Hadn't seen anything out of her until late August, and she was standing one day. There is a percentage of cows that don't conceive, and then there is another percentage that conceive and don't maintain the pregnancy for some reason. It may be a repro thing with the cow...it may not be her fault...maybe she got bumped just right by another cow? Regardless of the reason, the result is the same...open cows. We're usually left to try and guess why (if it matters) and then determine whether it makes any sense to carry that cow & make excuses for her until she gets back with the program. I've seen some that do and some that just don't.