The XSLT version of the detail view in the staff client has used a Bootstrap modal for a while now. The non-XSLT version still uses Greybox, and this should be corrected.
Isn't non-XSLT detail view deprecated anyway? http://wiki.koha-community.org/wiki/Coding_Guidelines#DEPR1:_non-XSLT_search_results_and_bib_display_templates_deprecated
They are deprecated but not removed yet - if it helps getting rid of some other code (greybox?), I would not be opposed to change the behaviour there.