Treat nodes without level tag as belonging to the level of the way it is a member of
Nodes that have other tags but no level tag but are part of another way should be treated as having implicitly the same layer as the way it is a member of.
For example in train stations there will be many nodes on the highway=railway: stop positions, railway signals and switches, milestones (and all the other railway-map-specific tags). If there are multiple levels, each highway=railway will have explicitly a level tagged. The mentioned nodes usually not, because they are simply part of the way.