The phrase "drop a kite" refers to the act of sending a message in a secretive way, often by using a kite dropped from a high place. However, there are several synonyms that can be used in place of this phrase. One alternative is "send a signal," which can refer to any form of covert communication. Another option is "pass on a message," which implies a more direct form of communication, but still with a level of secrecy. "Relay information" and "transmit a dispatch" also convey the sense of subtly conveying a message without being caught. Ultimately, the choice of synonym will depend on the specific context and tone desired.