Potentially we have a Marketo landing page and form which are being used to display in all devices including an app with pre-fill switched on. How does the pre-fill work if I have been sent an email, fill out the form on say a web browser and then go to the app. I'm assuming that the app will not show the pre-fill?
The pre-fill is based on the recognition of the lead, which itself is based on the existence of a cookie. The cookie are browser specific, so there is no way, when changing browser, that the lead can be recognized.
What would be the best method to pre-fill bearing in mind that the user is going to flip between the app and web browser?
You'd have to control all the entry and exit points between the two contexts. This is virtually impossible to cover in full, though some services have attempted to use heuristics based on timestamps and IP addresses to address the gaps. I've opined about the problems and promise with this in the past, if you check my comment history (probably 6+ months ago).