In the last trading session, U.S. stocks ended on a mixed note ahead of the inflation data. Among the top ETFs, SPY lost 0.1% and DIA added 0.1%, while QQQ moved 0.3% lower on the day.Two more specialized ETFs are worth noting as both saw trading volume that was far outside of normal. In fact, both these funds experienced volume levels that were more than double their average for the most-recent trading session. This could make these ETFs the ones to watch out for in the days ahead to see if this trend of extra-interest continues.DEF: Volume 7.10 Times AverageThis defensive ETF was in the spotlight as around 158,000 shares moved hands compared with an average of 25,000 shares a day. We also saw some price movement as DEF added 0.3% in the last session.The move was largely due to bouts of volatility and uncertainty in the stock market that raised the appeal for defensive investment and could have a big impact on defensive ETFs like the ones we find in this ETF portfolio. DEF has gained 4.8% over the past month and has a Zacks ETF Rank #3 (Hold) with a Medium risk outlook.PRFZ: Volume 3.66 Times AverageThis small-cap ETF was under the microscope as nearly 436,000 shares moved hands. This compares with an average trading volume of roughly 120,000 shares and came as PRFZ added 0.9% in the last trading session.The movement can largely be blamed on beaten-down prices after a massive sell-off early this year. PRFZ has gained 9.5% in a month’s time and has a Zacks ETF Rank #3 with a Medium risk outlook. Want key ETF info delivered straight to your inbox? Zacks’ free Fund Newsletter will brief you on top news and analysis, as well as top-performing ETFs, each week.Get it free >>Want the latest recommendations from Zacks Investment Research? Today, you can download 7 Best Stocks for the Next 30 Days. Click to get this free report Invesco QQQ (QQQ): ETF Research Reports SPDR S&P 500 ETF (SPY): ETF Research Reports SPDR Dow Jones Industrial Average ETF (DIA): ETF Research Reports Invesco Defensive Equity ETF (DEF): ETF Research Reports Invesco FTSE RAFI US 1500 SmallMid ETF (PRFZ): ETF Research Reports To read this article on Zacks.com click here. Zacks Investment Research