Large yellowish stains on inside walls

Maintenance

Good morning 

I own a mid terrace Victorian property with a plaque on outside property stating when houses were built, 1890.  Two years ago the outside of the building was painted a drab dirty white with paintwork and plaque painted black. New colour scheme very light grey and paintwork dark grey and painted over the plaque.  

This is when the problems started, I am not sure what the plaque is made of stone? sandstone?  The marks have started seeping through to  inside wall. On the outside wall there is a large stain underneath the plaque. I had contacted two decorators who did not know what to do or the source of problem. Does anybody know if there is a product to mask stain or any idea what it could be. 
Thank you Christine