How public is a public app?

If I deploy an app to and leave it as "public" so no authentication is required, I know that anyone who knows the URL can access it. But realistically what is the chance of it being visited by accident? For example, are there any bots or scrapers that will find it within minutes and post it somewhere? Will it be indexed by Google? Or will it only be visited by someone who deliberately types in the address or clicks on a link that has been provided to them.

The use case it that sometimes I want to test an app with non-trivial numbers of people who I don't want to force to get credentials. Leakage of the URL would be annoying but not catastrophic, so I'm wondering what the realistic risk is of just having a public app and sharing the link.