One option when setting up discovery is to do ping sweep of the target subnet(s). It has pros and cons. Reference page 5 of the LMS 4.1 (not just) for Dummies Guide.
OTOH if an device appears inactive until pinged, it must not be typically generating traffic to its default gateway (or that gateway is not managed by LMS) as that would be the normal source of information about the host (ip-mac correlation anyhow) in a default discovery type.
Hope this helps.