Hi,
half day/day hikes: I park my bike on the parking lot (Hiking areas). If they are a lot of tourists/hikers I lock the bike. If the hike starts from a major street I park it about 100 m on the hiking trail. If there is shop / ticket or ranger office I ask if they can have a look to it.
day/multi day hikes: I store my stuff at a campground/hostel where I stay one night after the hike
Including sight seeing I make this every second day - I never had bad experience. This holiday I parked the loaded bike in Split (Croatia) at the ferry port and visited 3 hours the city.
Thomas