Indications that a smartphone may have a malware infection or have spyware installed include poor battery life, dropped calls and call disruption, unusually large phone bills, data plan spikes, performance problems, and other unexpected behaviors, such as disappearing icons, mysterious icons that weren’t installed by the user, unexpected restarts, etc.¹

Reference:

[1] https://for585.com/nwoez (readwrite.com blog post on mobile malware signs).