I know we've addressed this a bit on the forum, but I thought it would be a good time to write an article on it as well.
Generally, YES, someone else can touch your magickal. Its inevitable at some point anyway.
When dealing with an entity or spirit, they can, for the most part, distinguish your own personal energies from the energies of others.
However, if someone is going to be handling your vessel for an extended period of time, say to be sized, or borrowed, it is suggested that you cleanse the vessel using a charging/cleansing box created by a professional so that your spirit/entity is not torn between energies.
And, if you like, you may perform the welcoming ritual again once you receive them back.
Also, do pay attention to your entities and spirits. Not all of them like to be touched by strangers.
The same goes with a magickally charged vessel. Any small amount of handling is OK, but if the handling is for an extended period of time, we do suggest a cleansing so that you, as the keeper, get the full blending of energies.
Most practitioners do take precautions to veil the energies of the vessels and magickals that they create from others, and although this does help, it does not prevent everyone from taking notice.