Toilet accessories like brushes and plungers are best kept close to where they are used. If your bathroom does not have an under the sink cabinet to store these items or you don’t want to place them there.
The toilet plunger holder (if your plunger has one) should be cleaned on a regular basis. Spray the exterior with a bathroom spray that contains bleach, let it sit for a few minutes and then rinse it clean in the shower or tub.
Always keep a plunger in the guest bathroom and put it in an obvious place that guests will be able to easily find it. Make sure there is a discreet, sanitary way for them to put it away after they use it too.