Who would that be? They put out a missing person bulletin before BM ever arrived back from Denver. The daughters were not at home.
Perhaps a neighbor knew her bike? Maybe someone in LE knew all 130 people who lived in Mayville and had seen a striking woman on that bike before? Heck, I know a lot of bikes in my neighborhood (I like bikes).
Has LE ever stated that they confirmed ownership? They said she "reportedly" was on a bike. As far as we know, there is no bike.
The only persons mentioning the bike seem to be BM, the nephew and, if you think his mom wrote the email to the church, his mother. But I wouldn't be surprised if the neighbor thought she could recognize the bike.
Some places require bikes to be licensed, as well (serial numbers registered with local authorities). We simply do not know what LE knows about the bike. But apparently, there is no bike there now and BM seems to believe his wife was on a bike when she went missing - so surely he must see that her bike is missing from the home, now that he's back there?
Why would LE show the bike to someone who they are carefully trying to keep out of the middle of their investigation? I am guessing that they asked BM to describe his wife's bike to them, not ever showing him the bike. They likely asked him its last known condition, etc.
I can't remember what else BM said about his phone call to the neighbor, but didn't he ask neighbor to check for car and bike? So surely the neighbor could have merely walked down her driveway and talked to arriving police, with a description of the bike.