The idea came from
ZBX-15591. It would be nice if Zabbix would set agent icon to green when data is received also from active agent. Additional agent parameter e. g. ActiveUnavailable could be used to define the limit of time without "active data" which would cause the Agent icon to go back to gray probably setting host availability to "unknown".
For an active agent a parameter (e. g. HeartbeatInterval) could be introduced making it constantly ping proxy/server.
Upd: as proposed by richlv below, the availability of Passive and Active agent could be visualized separately.