General aviation pilot reported receiving a response on Unicom frequency while inbound that indicated the tower at non-towered SGH airport was operational. Reporter stated charts do not depict the tower or frequencies and it appears to be operational only during military drone activity.

Narrative

I made a call on Springfield Unicom frequency when 5 miles out of this non towered field with my planned pattern entry advisory and intention to land. I was answered with 'Springfield Tower is active on 126.75'. I switched to this frequency and was eventually able to contact them and receive landing clearance. Their radio coverage was very spotty and they ended up requesting radio checks on both frequencies with poor results from both mine and another aircraft. The trouble is the airport is not depicted as having a Tower on the charts. Airport data does not indicate even a part time Tower and indeed the frequencies they were using for both Tower and Ground were not listed. There were NOTAMs for UAV activity and indeed they were flying several military drones during the time I was there. I got the impression the tower is active when the military is flying drones but I could find no confirmation of that. I consider this situation to be unsafe. While aircraft normally are on Unicom frequency and announce positions and intentions; this is not required at non towered fields. I feel that any Tower Control requires charting updates; frequency listing and active times be published. Part time Towers should require proper charting notation; NOTAM of active times and frequencies used.