vaillant boiler losing pressure

Hi I have a vaillant turbomax which is losing pressure very quickly over a period of 3-4 days.There are no external leaks.Can you suggest ways to detect a leak or could there be another problem.My local plumber bled all the radiators but the problem persists.HELP! HELP! HELP!
I find it very annoying to keep having to refill it. Your help is appreciated. Seedat
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.