The latest transactions raise its holding in the CCTV and video surveillance provider to just short of 17%.
Separate stock market filings show NPI first bought 25,000 shares to take it above 15% then snapped up a further 141,000 to give it 16.94% of IndigoVision.
Although the exact price paid for each share was not disclosed the investment is likely to have come in at around £470,000 based on IndigoVision's closing price of 288p on Wednesday.
NPI's total holding is worth around £3.7 million.
NPI first declared a holding in IndigoVision, which has it headquarters at the Edinburgh Technopole business park, in April this year and has steadily built up its position since then.